1. Field of the Invention
The present invention relates generally to compound retarders. More specifically, the present invention is directed to the use in display devices of achromatic compound retarders that exhibit an achromatic composite optic axis orientation and/or an achromatic composite compound retardance at each of at least two composite retarder orientation states. Further, the present invention is directed to the use of such achromatic compound retarders to create achromatic inverters for display applications.
2. Background of the Related Art
Liquid crystal retarders are increasingly utilized within optical devices such as tunable filters, amplitude modulators and light shutters. Planar aligned smectic liquid crystal devices function as rotative waveplates wherein application of an electric field rotates the orientation of the optic axis but does not vary the birefringence. In contrast, homeotropically aligned smectic liquid crystals, homogeneous aligned nematic devices, and nematic pi-cells function as variable retarders, wherein application of an electric field varies the birefringence. Chromaticity is a property of birefringent elements, both passive and active liquid crystals. There are two main components to chromaticity: (1) dispersion, which is the change in the birefringence (xcex94n) with wavelength xcex; and (2) the explicit dependence of retardance on 1/xcex due to the wavelength dependent optical pathlength. Both components contribute to increased birefringence with decreased wavelength. A birefringent material having a particular retardance at a design wavelength has higher retardance at shorter wavelengths and lower retardance at longer wavelengths. Chromaticity places limitations on the spectral operating range of birefringent optical devices.
Chromaticity compensation for passive retarders was addressed by S. Pancharatnam, Proc. Indian Acad. Sci. A41, 137 [1955], and by A. M. Title, Appl. Opt. 14, 229 [1975], both of which are herein incorporated by reference in their entirety. The wavelength dependence of passive birefringent materials can be reduced by replacing single retarders with compound retarders. The principle behind an achromatic compound retarder is that a stack of waveplates with proper retardance and relative orientation can be selected to produce a structure which behaves as a pure retarder with wavelength insensitive retardance. Pancharatnam showed, using the Poincare sphere and spherical trigonometry, that such a device can be implemented using a minimum of three films of identical retarder material. A Jones calculus analysis by Title (supra) verified the conditions imposed on the structure in order to achieve this result: (1) the requirement that the composite structure behave as a pure retarder (no rotation) forces the input and output retarders to be oriented parallel and to have equal retardance; and (2) first-order stability of the compound retarder optic axis and retardance with respect to wavelength requires that the central retarder be a half-wave plate. These conditions yield design equations that determine the retardance of the external elements and their orientation relative to the central retarder for a particular achromatic retardance. Because these design equations specify a unique orientation of the central retarder and a unique retardance for the external retarders, they have never been applied to active liquid crystal devices and the problem of active retarder chromaticity remains.
For the specific example of an achromatic half-wave retarder, the design equations dictate that the external retarders are also half-wave plates and that the orientation of the external retarders relative to the central retarder is xcfx80/3. By mechanically rotating the entire structure, wavelength insensitive polarization modulation is feasible. Furthermore, Title showed that the compound half-wave retarder can be halved, and one section mechanically rotated with respect to the other half to achieve achromatic variable retardance. Electromechanical rotation of such compound half-wave retarders has been used extensively to tune polarization interference filters for astronomical imaging spectrometers.
The primary application of ferroelectric liquid crystals (FLCs) has been shutters and arrays of shutters. In the current art, on- and off-states of an FLC shutter (FIG. 1) are generated by reorienting the optic axis of FLC retarder 10 between xcfx80/4 and 0 with respect to bounding crossed or parallel polarizers 20 and 22. In the off-state, x-polarized light is not rotated by the liquid crystal cell and is blocked by the exit polarizer. In the on-state, the polarization is rotated 90xc2x0 and is therefore transmitted by the exit polarizer.
For maximum intensity modulation, the cell gap is selected to yield a half-wave retardance at the appropriate design wavelength. The on-state transmission of x-polarized light is theoretically unity at the design wavelength, neglecting absorption, reflection and scattering losses. At other wavelengths the transmission decreases. The ideal transmission function for an FLC shutter as in FIG. 1 is given by                     T        =                                                            1                -                                                      sin                    2                                    ⁢                                      δ                    /                    2                                                                                      ON                                                      (                                  α                  =                                      π                    /                    4                                                  )                                                                        0                                      OFF                                                      (                                  α                  =                  0                                )                                                                        (        1        )            
where xcex4 is the deviation from half-wave retardance with wavelength. This expression indicates a second-order dependence of transmission loss on xcex4. The off-state transmission is in principle zero, but in practice it is typically limited to less than 1000:1 due to depolarization by defects, the existence of multiple domains having different alignments, and fluctuations in the tilt-angle with temperature.
High transmission through FLC shutters over broad wavelength bands is feasible for devices of zero-order retardance, but it is ultimately limited by the inverse-wavelength dependence of retardation and the rather large birefringence dispersion of liquid crystal materials. For instance, a visible FLC shutter device that equalizes on-state loss at 400 nm and 700 nm requires a half-wave retarder centered at 480 nm. A zero-order FLC device with this retardance, using typical FLC birefringence data, has a thickness of roughly 1.3 microns. The transmission loss at the extreme wavelengths, due to the departure from half-wave retardance, is approximately 40%. This significantly limits the brightness of FLC displays and the operating band of FLC shutters and light modulators. In systems incorporating multiple FLC devices, such as tunable optical filters or field-sequential display color shutters, this source of light loss can have a devastating impact on overall throughput and spectral purity.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.
This invention provides achromatic compound retarders, achromatic polarization switches, and achromatic shutters using the achromatic compound retarders. It further provides achromatic variable retarders utilizing smectic liquid crystals. An achromatic shutter according to this invention is demonstrated which provides excellent on-state transmission over the entire visible, xe2x89xa794% from 400 nm to 700 nm after normalization for polarizer loss, and high contrast, 1000:1 from 450 nm to 650 nm.
One embodiment of the achromatic compound retarder of this invention comprises a central rotatable smectic liquid crystal half-wave retarder and two external passive retarders positioned in series with and on either side of the liquid crystal retarder. The external retarders are equal in retardance and oriented parallel to each other. Design equations determine the retardance of the external elements and their orientation relative to the central retarder to obtain a particular retardance for the compound structure. A reflective version of the achromatic compound retarder described above is constructed with a smectic liquid crystal quarter-wave retarder positioned between a single passive retarder and a reflector.
In the achromatic compound retarders of this invention there is, in general, an orientation of the central retarder for which the structure has maximum achromaticity in both orientation and retardance. Important aspects of this invention are the discoveries that (1) the composite retardance at the design wavelength does not change when the optic axis orientation of the central retarder is changed and (2) there are optic axis orientations of the central retarder for which the optic axis orientation of the compound retarder is stable (achromatic) even though the composite retardance is not achromatic.
The central retarder may comprise a liquid crystal retarder, as described above. In the case of a smectic liquid crystal cell, application of an electric field rotates the optic axis between two or more orientations. One of the orientations provides maximum achromaticity of the compound retardance. As discussed above, there is also at least one other optic axis orientation for which the optic axis of the compound retarder is achromatic, even though the composite retardance is not. Furthermore, the composite retardance at the design wavelength does not change when the optic axis orientation of the smectic liquid crystal cell is changed.
The central retarder may also comprise a spatially switched planar-aligned passive retarder, in which the orientation of the optic axis varies as a function of position on the spatially switched passive retarder. The spatially switched passive retarder has at least two optic axis orientations states, with one of the orientations causing the retardance of the compound retarder to be substantially achromatic, and the second orientation causing the optic axis orientation of the compound retarder to be substantially achromatic, even though the composite retardance may not be.
The achromatic properties discussed above are utilized in the achromatic polarization switch of this invention, comprising a linear polarizer and the compound achromatic retarder, and in the achromatic shutter of this invention, comprising the compound achromatic retarder positioned between a pair of polarizers. In one optic axis orientation state of the central retarder (the xe2x80x9cON-statexe2x80x9d) the compound retarder is achromatic and in a second optic axis orientation state of the central retarder (the xe2x80x9cOFF-statexe2x80x9d) the compound retarder is oriented parallel to one polarizer and the light therefore does not xe2x80x9cseexe2x80x9d the compound retarder. In the off-state, fixed retardance with wavelength is therefore not necessary. Providing achromatic orientation of the compound retarder in the off-state yields high contrast shutters. Reflection-mode shutters are further provided in this invention.
In alternative liquid crystal compound retarder embodiments, the rotatable smectic liquid crystal half-wave retarder is replaced by first and second liquid crystal variable birefringence retarders. The first and second variable birefringence retarders have first and second fixed optic axis orientations, respectively, and retardances which can be switched between zero and half-wave. In operation, when one retarder is switched to zero retardance, the other is switched to half-wave, and vice-versa, so that the composite retardance of the pair is a half-wave retardance with orientation switchable between the first and second optic axis orientations.
The achromatic variable retardance smectic liquid crystal compound retarder of this invention comprises an active section rotatable with respect to a passive section. The active section comprises two liquid crystal retarders: a half-wave plate and a quarter-wave plate oriented at angles xcex12 and xcex12+xcfx80/3, respectively, where the angle xcex12 is electronically switchable. The passive section comprises two retarders: a quarter-wave plate and a half-wave plate oriented at angles xcex11 and xcex11+xcfx80/3, respectively, where the angle xcex11 is fixed. The quarter-wave plates are positioned between the half-wave plates. The composite retardance of the compound structure is 2(xcfx80/2xe2x88x92xcex12+xcex11). To vary the retardance, the liquid crystal retarders in the active section are both rotated.
The planar-aligned smectic liquid crystal cells of this invention have continuously or discretely electronically rotatable optic axes. The smectic liquid crystal cells can utilize SmC* and SmA* liquid crystals, as well as distorted helix ferroelectric (DHF), antiferroelectric, and achiral ferroelectric liquid crystals. The variable birefringence liquid crystal cells of this invention can include homogeneously aligned nematic liquid crystals, pi-cells, and homeotropically aligned smectic liquid crystal cells.
The present invention may be achieved in whole or in part by an achromatic compound retarder that exhibits a compound retardance and a compound optic axis, comprising: (1) a first passive retarder unit having a predetermined retardance at a design wavelength, and having a predetermined optic axis orientation; (2) a second passive retarder unit having the same retardance as the first passive retarder unit at the design wavelength, and having substantially the same optic axis orientation as the first passive retarder unit; and (3) a central retarder unit positioned between the first and second retarder units, the central retarder unit having a retardance xcfx80 at the design wavelength, and having an optic axis orientation that varies as a function of position on the central retarder unit, wherein the optic axis orientation varies between at least a first orientation state, in which the compound retardance is substantially achromatic, and a second orientation state.
The present invention may also be achieved in whole or in part by a reflection mode achromatic compound retarder, comprising: (1) a first passive retarder unit having a predetermined retardance at a design wavelength, and having a predetermined optic axis orientation; (2) a reflector; and (3) a spatially switched retarder unit positioned between the first retarder unit and the reflector, the spatially switched retarder unit having a retardance xcfx80/2 at the design wavelength, and having an optic axis orientation that varies as a function of position on the central retarder unit, wherein the optic axis orientation varies between at least a first orientation state, in which the compound retardance is substantially achromatic, and a second orientation state.
The present invention may also be achieved in whole or in part by an achromatic compound retarder that exhibits a composite optic axis orientation and a composite retardance, comprising: (1) a first passive retarder unit having a predetermined retardance at a design wavelength, and having a predetermined optic axis orientation; (2) a second passive retarder unit having the same retardance as the first passive retarder unit at the design wavelength, and having substantially the same optic axis orientation as the first passive retarder unit; and (3) a central retarder unit positioned between the first and second retarder units, the central retarder unit having a retardance xcfx80 at the design wavelength, and having an optic axis orientation that switches between at least two orientation states as a function of position on the central retarder unit, wherein the composite optic axis orientation and/or the composite retardance is substantially achromatic at two orientation states of the central retarder unit.
The compound retarder according to the invention can also be employed to provide a novel achromatic inverter in a reflective or transmissive type display. The achromatic inverter works in combination with a liquid crystal display panel to provide four states of intensity or brightness, two high and two low, so that the reflective or transmissive display is capable of displaying an inverse image frame.
In particular, in accordance with one embodiment of the invention, a reflective display comprises one or more retarders having in-plane retardance and in-plane orientation, at least one of the retarders being an active retarder, and a ferroelectric liquid crystal display. The one or more retarders work in combination with the ferroelectric liquid crystal display to provide four states of brightness.
In accordance with another embodiment of the invention, a reflective display comprises a linear polarizer, an actively controlled liquid crystal retarder and a ferroelectric liquid crystal display. In accordance with a further embodiment, a reflective display comprises a polarizing beam splitter, an actively controlled liquid crystal retarder and a ferroelectric liquid crystal display. In both embodiments, the actively controlled liquid crystal retarder and the ferroelectric liquid crystal display are both switchable between at least two orientations to provide four states of brightness. In accordance with still another embodiment, a transmissive display comprises a first linear polarizer, a first actively controlled liquid crystal retarder and a ferroelectric liquid crystal display, a second actively controlled liquid crystal retarded and a second linear polarizer.
The active retarder can be either a smectic or a nematic liquid crystal retarder. In the accordance with another embodiment of the invention, a reflective display comprises a linear polarizer, an actively controlled nematic liquid crystal retarder and a ferroelectric liquid crystal display. In accordance with a further embodiment, a reflective display comprises a polarizing beam splitter, an actively controlled nematic liquid crystal retarder and a ferroelectric liquid crystal display. In both embodiments, the actively controlled nematic liquid crystal retarder and the ferroelectric liquid crystal display are both switchable between at least two orientations to provide in combination four states of brightness. Additionally, a passive retarder can be provided between the actively controlled nematic liquid crystal retarder and a ferroelectric liquid crystal display. Further, the actively controlled nematic liquid crystal retarder can comprise one or more pi-cells. Where one or more pi-cells are employed as the actively controlled nematic liquid crystal retarder, a passive retarder can be located between the one or more pi-cells and the ferroelectric liquid crystal display or between adjacent pi-cells. Further, in addition to the one or more pi-cells, the display may include additional actively controlled liquid crystal retarders, arranged adjacent to the one or more pi-cells or in between the one or more pi-cells.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.